1. Field of Invention
This invention is generally related to an image forming apparatus which uses multi-beam raster output scanners (ROS) to form images on a medium.
2. Description of Related Art
Prealigned dual and quad laser diodes are very expensive. While prealigned dual laser diodes are desirable in xerographic based electronic printers and copiers, due to cost considerations, individual laser diodes are normally used. FIGS. 1 and 2 illustrate top and side views, respectively, of a conventional rotating polygon-based optical system 100 and a known rotating polygon 140. It should be appreciated that the functions described below equally apply to most rotating polygon-based systems, independently of the number of light sources used.
As shown in FIGS. 1 and 2, the ROS optical system 100 includes a pair of sagittally offset laser diodes 102 and 103 that emit laser beams 121 and 123, respectively. The laser beams 121 and 123 emitted by the laser diodes 102 and 103 are collimated by a collimator lens 110. A sagittal aperture stop 120 is placed in a position where the laser beams 121 and 123 cross the system optical axis 500, to control the aperture size, which in turn controls the spot size on the photoreceptor image plane 182. The input cylinder optical elements 130 and 131 focus the laser beams 121 and 123 on the surface of the current polygon facet 144 of the rotating polygon 140. After reflecting from the current polygon facet 144, the laser beams 121 and 123 pass through the Fxcex8 lens 150. The Fxcex8 lens 150, in general, has relatively low power in the tangential meridian. The Fxcex8 lens 150 focuses the laser beams 121 and 123 in the tangential meridian to control the scan linearity in terms of uniform spot displacement per unit angle of polygon rotation. A sagittal aperture stop 160 is placed in a position where laser beams 121 and 123 again cross the system optical axis 500.
A motion compensating optical element (MCO) 170 then reimages the focused laser beams 121 and 123 from the current polygon facet 144 onto the photoreceptor image plane 182 at a predetermined position, independently of the polygon angle error or tilt of the current facet 144. Such compensation is possible because the focused laser beams 121 and 123 are stationary xe2x80x9cobjectsxe2x80x9d before the Fxcex8 lens 150 and the motion compensating optical (MCO) element 170. Although, due to a polygon tilt or wobble, the laser beams 121 and 123 are reflected to different positions of the post polygon optics aperture for each different facet of the rotating polygon, the beams 121 and 123 are imaged to the same position on the photoreceptor image plane 182.
In rotating polygon, ROS-based xerographic copiers and printers, distortions occur from several sources of beam spacing errors. The sources of beam spacing errors in multi-beam rotating polygon based optical systems illustrated in FIG. 2 are optical and/or mechanical in nature. Beam spacing errors fall into one of the following categories: residual errors in the nominal design, thermal effects, vibration, and fabrication and wear errors in the various optical and mechanical components in the system.
Nominal differential bow is a source of residual beam spacing error. Even if the components were perfectly fabricated and assembled, beam-to-beam differential bow error will be present because the optical design cannot completely eliminate image distortion, as illustrated in FIGS. 3 and 4. Variations in ambient temperature produce changes in the refractive index, position, and thickness of optical components. These changes cause differences in scan line shape and position, as shown in FIGS. 5 and 6. Mechanical vibrations result in changes in scan line position, which can lead to beam spacing error.
FIGS. 3-6 illustrate the various types of errors which can be introduced by differential scan line bow. FIG. 3 shows a barrel type bow distortion. Specifically, FIG. 3 shows the center of curvatures of a pair of bowed scan lines 185 and 187 located on opposite sides of an ideal scan line 189 in such a fashion that the bowed scan lines create a barrel distortion. This occurs whether the bowed scan lines 185 and 187 have the same or different radius of curvature.
FIG. 4 shows a pin cushion type bow distortion. Specifically, FIG. 4 shows the center of curvature of the bowed scan lines 185 and 187 are also on the opposite side of the ideal scan line 189 (with the same or different radii). However, the arrangement of the bowed scan lines 185 and 187 relative to each other forms a pin cushion distortion. Again, this occurs whether the bowed scan lines 185 and 187 have the same or different radii of curvature.
FIG. 5 shows the ideal scan line 189 as a dashed line. In FIG. 5, first bowed scan line 187 has a first radius of curvature which is different from the radius of curvature of the second bowed scan line 185.
FIG. 6 shows bowed scan line 185 superimposed over the bowed scan line 187. As shown in FIG. 6, the bowed scan line 185 has a center of curvature which is on the opposite side of the ideal scan line 189 from the center of curvature of the bowed scan line 187. As can be seen from FIGS. 3-6, the bow appears as a displacement of a scan line in the process direction as the line extends in the fast scan direction.
As shown in FIG. 7, there are shown a plurality of dashed lines representing ideal raster scan line paths 175 across a photoreceptor. The scan line spots 121xe2x80x2 and 123xe2x80x2 and 121xe2x80x3 and 123xe2x80x3, are shown with respect to each other and with respect to the ideal scan line path 175. Ideally, the raster scan line spots 121xe2x80x2, 123xe2x80x2, 121xe2x80x3 and 123xe2x80x3 travel across the photoreceptor within the corresponding ideal scan line paths 175. However, due to the factors discussed above, the raster scan line spots 121xe2x80x2, 121xe2x80x3, 123xe2x80x2, and 123xe2x80x3 often, if not usually, do not travel within the ideal scan line paths 175.
As can be seen on the left side of FIG. 7, the raster scan spots 121xe2x80x3 and 123xe2x80x3 are separated from each other by a distance Y and do not lie within ideal scan line paths 175. On the right side of FIG. 7, the raster scan spots 121xe2x80x2 and 123xe2x80x2 overlap by a distance X. It should be appreciated that, due to bow and the like, as the raster scan spots 121xe2x80x2,121xe2x80x3,123xe2x80x2, and 123xe2x80x3 move across the photoreceptor, the distortions shown in FIGS. 3-6 develop.
Fabrication variations in material parameters, component geometry, and assembly, manifested in misalignment, improper beam conditioning and defocusing, result in both uniform and non-uniform variation of the beam spacing across the image plane. Local variations in the photoreceptor and tilt errors among the various facets 141-148 of the polygon mirror 140, for example, produce variation in process direction beam position from scan to scan. Curvature error in the lenses can produce either a widening or narrowing of the distance between scanning beams. All of the optical elements of a multi-beam rotating polygon-based optical system 100 may therefore introduce a degree of beam-to-beam spacing error. The combination of errors creates an error unique to each machine, and is commonly referred to as the constant beam-to-beam spacing error.
It also should be appreciated, however, that the constant beam-to-beam spacing error is constant over a limited time period, such as that of several scans to that of hours, days or even longer. That is, the constant beam-to-beam spacing error slowly changes over time. The component parts of the multi-beam rotating polygon-based optical system 100 and the assembly tolerances of those parts tend to slowly deteriorate over time, thus imparting a variable quality to the otherwise constant beam-to-beam spacing error. Consequently, it is more accurate to describe the constant beam-to-beam spacing error as a semi-static beam-to-beam spacing error.
Thus, in the conventional multi-beam rotating polygon-based optical system 100 described above, the scan lines usually either improperly overlap or are excessively spaced apart. The raster scan shown in FIG. 7, illustrates beam-to-beam spacing and overlap errors for two different sets of dual laser diodes resulting from either differential scan line bow and/or the constant or semi-static errors. Optical system designs can incorporate compensators or adjustments to correct for this error type, but in many cases residual errors persist even after correction has been implemented.
This invention provides systems and methods for detecting beam-to-beam spacing errors on the image plane of a photoreceptor during operation of a xerographic based electronic printer.
This invention separately provides systems and methods for automatically adjusting for beam-to-beam spacing errors on the image plane of a photoreceptor during operation of a xerographic based electronic printer.
This invention separately provides systems and methods for measuring average density variations in test patterns representative of raster scan line spacing errors on the image plane of a photoreceptor during operation of a xerographic based electronic printer.
This invention separately provides systems and methods to enable a xerographic printer user to obtain an image without objectionable banding artifacts.
In various exemplary embodiments, the systems and methods of the invention provide for specific beam-to-beam spacing error adjustments so that residual errors do not remain even after adjustments have been made. If a first adjustment is not sufficient to fully correct the beam-to-beam spacing errors, in various exemplary embodiments, the systems and methods of the invention are designed to reevaluate the beam-to-beam spacing errors to reduce, or ideally remove, such residual errors.
In various other exemplary embodiments, the systems and methods of this invention use a conventional rotating polygon based optical system, gray level measurement devices, a controller and means to measure and possibly adjust for beam-to-beam spacing errors.
In various exemplary embodiments, the apparatus of this invention uses a conventional rotating polygon-based optical system having two or more light sources and several optical elements. One or more of the various light sources and/or optical elements are adjustable in response to an error signal generated by the controller in view of signals received from one or more gray level measurement devices.
In various exemplary embodiments, two or more sensors of a gray level measurement device are located at fixed positions along the axial length of the photoreceptor. In various other exemplary embodiments, the apparatus includes a single gray level measurement device that is movable along between the ends of the photoreceptor. The movable sensor of the gray level measurement device can detect developed mass per unit areas for the full width of the photoreceptor. In various other exemplary embodiments, the gray level measurement device includes two sensors located relative to the width of the photoreceptor. In this case, each sensor can be moved over a portion of the photoreceptor. Each sensor detects a developed mass per unit area of a viewed area on the photoreceptor. Each sensor generates a signal corresponding to the developed mass per unit area in the viewed area and sends the signal to the controller.
The controller generates a beam-to-beam spacing error signal based on the sensor signals and determines which optical element or laser diode can be adjusted to adjust for the error that occurs in the viewed area. The controller signal is then sent to one or more appropriate optical elements and/or light sources, implementing the adjustment.
In various exemplary embodiments, the gray level measurement devices are implemented using enhanced toner area coverage sensors.
In various other exemplary embodiments, the apparatus includes a conventional polygon-based optical system having four light sources and several optical elements. The four light sources may be implemented as a combination of 4 single light sources or 2 double light sources. In various other exemplary embodiments, the apparatus includes a conventional polygon-based optical system having 2 single light sources and several optical elements.
In various other exemplary embodiments, the apparatus includes more than gray level measurement devices.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention.